The present invention is directed to a suspension system for a vehicle comprising displacer means which can be actuated by the relative motions between a vehicle wheel and the vehicle body, the working space of the displacer means containing a flow medium and being connected by at least one throttle means to at least two spring means which can be loaded by the flow medium.
Conventional versions of such suspensions as shown in German Auslegeschrift (hereinafter DE-AS) No. 1,430,836 or German Offenlegungschrift (hereinafter DE-OS) No. 1,655,029, feature two spring means apiece in the form of hydropneumatic springs which are connected to a displacer means. With these systems, the problem of damping of the vehicle body, increases considerably with loading of the vehicle and is not satisfactorily solved in that the damping of the wheels is not as uniform as possible with changing vehicle loads.
In suspension systems of the aforementioned type with, for example, two hydropneumatic springs per vehicle wheel, a proportionally uniform relative change of volume corresponds to any static pressure change .DELTA.P.sub.static in any spring because of the formula p.times.v=constant, which is generally a valid statement. However, this means that the ratio of the sub-volume flows displaced by the displacer means to the two springs is always the same: ##EQU1##
Here the indices "a" stand for empty, "b" for loaded and "1" for the gas volume of the first spring and "2" for the gas volume of the second spring. From the above-explained relationships, it follows that throttles which are placed in front of one or both springs will always make the same proportional contributions to the overall damping of the system. Therefore, a load-dependent change in damping cannot be achieved.
With suspension and damping systems for motor vehicles of the type referred to above, it has been necessary up till now to strike compromises in the damping of the suspension, as in the use of a mechanical suspension together with conventional shock absorbers. If the damping is tailored to an average vehicle load, then it will be preceived as too hard at small loads and too soft when large additional loads are imposed.
If in the determination of the damping characteristic, even greater emphasis is placed on the fundamental (natural) frequency of the wheel, which lies at approximately 12 Hz, there exists the danger that adequate damping will be completely impossible in the range of the body's fundamental (natural) frequency of approximately 1.2 Hz. This means, for example, that in addition to special comfort adjustments, sports adjustments, i.e., a firm yet positive suspension, must also be offered.
Many attempts to vary damping approximately as a function of load, travel or even frequency have already been undertaken.
In the case of travel-dependent damping variations, a larger number of throttle openings can be cleared or covered with, for example, an adjusting piston. The suspension system disclosed in DE-OS 2,655,705 creates a way of bringing additional auxiliary damping pistons into play beyond a certain rebound or compression travel. In Austin vehicles provision is made for a travel-dependent change in the displacer surface, while a change in displacer speed brought about by a change in the transmission ratio between wheel stroke and damper stroke has already been proposed as well. Travel-dependent dampers have the disadvantage that in the case of level-controlled vehicles the damping force does not increase with load since the vehicle height or clearance remains constant.
Damping in hydropneumatic suspensions which is variable as a function of pneumatic spring pressure is also known as evidenced by DE-OS Nos. 1,555,382 or 1,555,383, but all of these solutions require expensive control devices which are susceptible to malfunction because of the danger of fouling and wear. In these devices the positioning or control element in its turn must be damped to avoid uncontrolled switching transients. This again leads to greater temperature dependency.
Finally, frequency-dependent changes in the throttle cross-sections have also been proposed as indicated in DE-AS 1,045,256 or DE-OS No. 1,555,491. The frequency-sensitive inertial masses used therein give rise, on an even larger scale, to the above-mentioned disadvantage of a great temperature dependency since particularly strong inherent damping of the positioning elements is required because of the rapid pressure pulsations.
In addition, German Patentschrift No. 1,675,634 or DE-OS No. 3,406,835 show a pneumatic spring and damper system, the operation of which is load-dependent and frequency-dependent, but in both very large quantities of air are needed, by comparison, in order to be able to achieve adequate damping. Even at relatively low displacement speeds, the damper forces decrease significantly and the overall spring rate is significantly increased. This causes a rapid rise in the body frequency.
German DE-OS No. 2,017,098, shows a hydropneumatic suspension system with automatic leakage readjustment in which the oil chamber of the displacer is connected to two gas volumes, i.e., the actual pneumatic spring volume and the adjustment gas volume of an adjustment element. A throttle is also placed in front of the adjustment element in this case (FIG. 5), but this throttle is placed in front of the more progressive spring of the two springs if it is (theoretically) assumed that the adjustment element is to assume a significant portion of the spring work at all. However, this is not the case according to the description found in this document. This throttle is undesirable, however, in that it becomes less and less effective as the load on the system increased.